Rattling question

Hey guys,

I pinpointed my rattling (i think). It is around the area of the license plate lights on the trunk lid. I used some weather stripping to outline the lights and plastic casing, and it helped a bit. However, now the problem lies in the two pieces of plastic that are hitting each other when the bass hits. It is the piece over the lights and the piece behind the lights. Its about a few milimeters in depth, so i'm not sure what i should do or what to put between the two pieces in order to stop the rattling.

I have 70sq ft of brown bread en route to my house I plan on matting the entire car with for sound deadening, but I don't think that will help 100% the rattling - unless it completely stops the bass from getting to the top of the trunk (which is the plan), in which case it would help. Anyone have any suggests? Is that foam stuff a good idea? If anyone has advice, please let know!